1. Field of the Invention
The present invention relates to an optical element manufacturing method suitable for manufacturing a nonlinear optical crystal element to be employed in a second harmonic laser, and a wafer.
2. Description of the Prior Art
The applicant of the present patent application proposed previously a laser as shown in FIG. 1 capable of stably emitting a laser beam in Japanese Patent Application No. Hei 3-17068 (hereinafter referred to as "cited reference"). Referring to FIG. 1, a pumping laser beam emitted by a laser diode 1 impinges on lens 2, travels through a concave mirror 3 and a quarter-wave plate 4, and impinges on a laser medium 5, such as a Nd:YAG laser medium. Upon the reception of the pumping laser beam, the laser medium 5 emits a fundamental laser beam LA(.omega.). Then, the fundamental laser beam LA(.omega.) travels through a nonlinear optical crystal element 6 and impinges on a plane mirror 7. The fundamental laser beam reflected by the plane mirror 7 travels through the nonlinear optical crystal element 6 and falls again on the laser medium 5. The fundamental laser beam travels to the left, as viewed in FIG. 1 through the quarter-wave plate 4 falls on the concave mirror 3 and is reflected by the concave mirror 3. The fundamental laser beam reflected by the concave mirror 3 travels through the quarter-wave plate and impinges again on the laser medium 5.
Thus, the fundamental laser beam shuttles between the concave mirror 3 and the plane mirror 7. The concave mirror 3, the quarter-wave plate 4, the laser medium 5, the nonlinear optical crystal element 6 and the plane mirror 7 form a laser resonator 8. A KTP (a single-axis uniaxial KTiOPO.sub.4 crystal) generates a second harmonic laser beam LA(2.omega.) of a frequency twice that of the fundamental laser beam LA(.omega.) by type II phase matching. The plane mirror 7 reflects the fundamental laser beam LA(.omega.) substantially entirely and transmits the second harmonic laser beam LA(2.omega.) substantially entirely. Consequently, the laser resonator 8 emits the second harmonic laser beam.
It is expected that the oscillation of polarized light of a mode nearest to the peak of gain occurs in a homogeneous line broadening laser, such as a solid-state laser, and single mode oscillation occurs due to the saturation of gain. However, actually, multimode oscillation occurs, in some cases, due to the effect of spatial hole burning; that is, when a standing wave a is generated by the laser resonator 8, an oscillation b of a different mode occurs as shown in FIG. 2 because the gain is not saturated sufficiently at the nodes of the standing wave.
The plane 6a of incidence of the nonlinear optical crystal element 6 on which the fundamental laser impinges of the laser previously proposed in the cited reference is inclined to the optical axis LA1 of the fundamental laser beam to suppress the effect of spatial hole burning. When the plane 6a of incidence of the nonlinear optical crystal element 6 is thus inclined to the optical axis LA1, the effective optical path length of the nonlinear optical crystal element 6 can be accurately adjusted to a predetermined value by adjusting the position of the nonlinear optical crystal element 6 with respect to a direction indicated by the arrow T perpendicular to the optical axis LA1. The position of the nonlinear optical crystal element 6 is thus adjusted so that the quantity of double refraction caused by the nonlinear optical crystal element 6 is exactly 90.degree.. Naturally, the plane 6b of exit of the nonlinear optical crystal element 6, as well as the plane 6a of incidence, may be inclined to the optical axis LA1.
The effect of adjusting the quantity of double refraction, i.e., phase difference, caused by the nonlinear optical crystal element 6 to 90.degree. will be described hereinafter. The fundamental laser beam emitted by the laser medium 5 is a circularly polarized laser beam. The fundamental laser beam is changed to a linearly polarized laser beam by the quarter-wave plate 4. The linearly polarized fundamental laser beam is reflected by the concave mirror 3 so as to travel through the quarter-wave plate 4, the linearly polarized fundamental laser beam changes again to a reverse-circularly polarized fundamental laser beam. Then, the reverse-circularly polarized fundamental laser beam travels through the laser medium 5 and impinges on the nonlinear optical crystal element 6. Since the quantity of double refraction of the nonlinear optical crystal element 6 is adjusted accurately to 90.degree., a linearly polarized fundamental laser beam goes out of the nonlinear optical crystal element 6. The linearly polarized fundamental laser beam is reflected by the plane mirror 7 and travels again through the nonlinear optical crystal element 6, whereby the linearly polarized laser beam changes again to the original circularly polarized laser beam. Thus, the fundamental laser beam that travels through the laser medium 5 is always reverse-circularly polarized, the spatial hole burning effect is suppressed by what is called a twist mode effect, which is described in, for example, Applied Optics, Vol. 4, No. 1 (January 1965).
Although the laser employing the nonlinear optical crystal element 6 having the inclined plane 6a of incidence or the inclined plane 6b of exit is proposed in the cited reference, nothing about a method of manufacturing such a laser is proposed in the cited reference.